Human eyes are less sensitive to color changes due to different color temperatures of room lighting. This feature is generally called chromatic adaptation. For example, when one moves from a room with fluorescent lighting which appears bluish, because of a high color temperature, to a room with incandescent lighting which appears yellowish, because of a low color temperature, the white walls of the room initially appear yellowish but appear whitish after a while. Due to such a chromatic adaptation feature of the human visual system, the color of the same image displayed on a television appears different when the color of room lighting is different.
With the recent trend toward higher resolution of liquid-crystal-display televisions, there is an increasing need for the function of finely adjusting the color cast of images in accordance with the type of room lighting, which makes the images look natural regardless of differences in the color temperature of room lighting. Because of this, the installation of a color sensor that detects room color temperature into liquid-crystal-display televisions is being promoted in order to detect room color temperature and automatically control the color cast of images in accordance with chromatic adaptation of the eye. For liquid crystal displays installed in portable devices such as smartphones and tablet PCs (personal computers), surrounding lighting changes continuously depending on the viewing and listening location and thus a sensor that automatically detects color temperature, such as a color sensor, becomes increasingly important.
This color sensor is configured to independently sense the spectra of R (red), G (green), and B (blue), which are three primary colors in the visible light region, in ambient light (the color sensor is hereinafter referred to as an RGB sensor).
The RGB sensor uses plural photoelectric conversion elements in order to sense ambient light, and a device serving as the photoelectric conversion element typically includes a photodiode. These photodiodes themselves cannot identify a color but can only detect the intensity of light (the amount of light). When an image is converted into electrical signals, color signals are acquired with the photodiodes in which each photodiode is covered with a color filter in order to distinguish a color and detecting the amount of light of the three primary colors, namely, R (red), G (green), and B (blue), with each photodiode.
An RGB sensor in the related art typically includes color filters that transmit or reflect only light having a specific wavelength based on interference of light or based on light shielding by absorption of light by a metal material in order to divide ambient light into the three primary colors of light, namely, R (red), G (green), and B (blue).
There is an increasing number of two-dimensional solid-state imaging devices, such as digital still cameras and video cameras, in which an object is captured and converted into an image by a photoelectric conversion element composed of a two-dimensional solid-state image sensor. A CCD (charge coupled device) image sensor and a CMOS (complementary metal oxide semiconductor) image sensor, which are the current mainstream solid-state image sensors, are provided with, on their pixels, R (red), G (green), and B (blue) color filters serving as on-chip filters and are provided with an infrared-cut filter on the package in order to block infrared light, which cannot be blocked by these color filters.
However, the RGB sensor in the related art requires three types of photomasks in order to form color filters that separate three primary colors RGB of light. The requirement of three types of photomasks increases the time and costs of the production process.
To reduce time and costs, there has been proposed a wavelength-selective filter, in place of the color filters, based on the anomalous transmission phenomenon of light due to surface plasmon resonance in which surface plasmons are excited by causing light to enter a periodic structure formed by subjecting a metal thin film to nano-scale micromachining.
A wavelength-selective filter (hereinafter referred to as a plasmonic filter) using the surface plasmon resonance is described in detail in PTL 1. There are various methods for inducing the anomalous transmission phenomenon. For example, there is a method, as shown in FIG. 9, in which an optical filter layer 500 is formed by forming a metal film 501 as thin as about 50 to 200 nm and pattering the metal film 501 so as to have an array of fine holes 502, 502, 502, . . . at a period that depends on the transmission wavelength. FIG. 10 shows the waveform of the spectrum that is transmitted through the optical filter layer 500 when light enters the optical filter layer 500. The surface plasmon resonance occurs due to the resonance between near field light generated by incident light and surface plasmons generated in the interface between a certain conductive material film and a dielectric film. To efficiently induce the surface plasmon resonance, the conductive material film and the dielectric film preferably have a homogeneous structure (uniform in physical properties such as material and refractive index, uniform in hole period and hole shape), and the dielectric film preferably has an optical property of being non-dispersive. The term conductive material film refers to a film that is formed of a metal element that is conductive by itself, has a reflectance of 70% or more in a given wavelength band, and is solid at room temperature, and refers to a film that is formed of an alloy or oxide of the metal element (see NPL 1).
For example, as the metal material, a material selected from the group consisting of aluminum, copper, silver, gold, titanium nitride, zirconium nitride, nickel, and cobalt, and alloys thereof is used.
In particular, aluminum and an alloy of aluminum and copper may often be used because of the following advantages:
(i) the plasma frequency is in the ultraviolet range, and thus the resonance phenomenon occurs in the long-wavelength region beyond visible light;
(ii) aluminum and an alloy of aluminum and copper are materials used in an ordinary semiconductor process and eliminate the need for special devices and materials in view of process integration;
(iii) the materials are inexpensive; and
(iv) the production process is simple and optical filters corresponding to different wavelengths are formed together.
Metal oxide transparent conductive materials, for example, In2O3-based materials typified by ITO (Sn:In2O3), ZnO-based materials typified by AZO (Al:ZnO), GZO (Ga:ZnO), BZO (B:ZnO), and IZO (In:ZnO), and IGZO-based materials, all of which have a plasma frequency in the range from visible light to near-infrared rays, are also used as materials of surface plasmonic filters because these materials, which have a plasma frequency lower than that of the above metals, are transparent under visible light.
In order to form an RGB sensor including the above plasmonic filters, periodic structures that are each designed to transmit R (red), G (green), or B (blue) color need to be disposed adjacent to each other.
According to NPL 2, when filters are formed to have a thickness of 150 nm by using Al as a metal material and have a periodic structure with a circular hole array, as shown in FIG. 11, the inter-hole periods a are required to be 260 nm for B (blue, wavelength 450 nm) light, 340 nm for G (green, wavelength 550 nm) light, and 420 nm for R (red, wavelength 650 nm) light in order to permit transmission of B (blue), G (green), and R (red) light. The wavelength λ of light to be transmitted and the inter-hole period (lattice constant) a have a linear relationship as shown in FIG. 12. The inter-hole period a varies depending on the metal material and the material of an insulating film around the metal material.
An increase in the number of pixels on CCD image sensors or the like involves a reduction in the size of each pixel and also a reduction in the distance between adjacent pixels. However, as described in PTL 2, filters with different periods need to be formed separately so that each region transmits light having a desired wavelength. If these filters are arranged in a stacking manner, problems arise such that, for example, the filters transmit light having plural wavelengths and the wavelength selectivity decreases and/or the transmissivity at a certain wavelength decreases.
Even if these filters are arranged separately, the following problems arise unless the distance between the filters is set appropriately. An example of the arrangement is shown in FIG. 13 and described. Plasmonic filters 102, 103, and 101 that respectively transmit light having three wavelengths, namely, R (red), G (green), and B (blue), are arranged as shown in FIG. 13. In the plasmonic filter 101, holes are uniformly arranged at an inter-hole period P1. Similarly, holes are uniformly arranged at an inter-hole period P2 in the plasmonic filter 102 and holes are arranged at a constant inter-hole period P3 in the plasmonic filter 103.
When these plasmonic filters 101, 102, and 103 are independently provided, there is only the inter-hole period that permits the transmission of each color of light and a transmission spectrum waveform with good wavelength selectivity is accordingly obtained. However, when the plasmonic filters 101, 102, and 103 are arranged as shown in FIG. 13, in the plasmonic filter 101 and the plasmonic filter 102, the interval a between a hole in the plasmonic filter 101 and a hole in the plasmonic filter 102 is a period a, which is different from the periods P1 and P2, and a small amount of light having wavelengths corresponding to the period a will be transmitted. The same applies to a period b between a hole in the plasmonic filter 102 and a hole in the plasmonic filter 103.
An example of the related art that solves this problem has been proposed in PTL 3. In PTL 3, a period a is made as close as possible to a period P1 of a plasmonic filter 101 and a period P2 of a plasmonic filter 102, so that unintentionally transmitted light has a wavelength closer to a desired wavelength. Specifically, the period a is set to 0.75 to 1.25 times (1.00±0.25) the periods P1 and P2.